Download Types of Rocks

Types of Rocks
Rocks are not all the same!
The three main types, or classes, of rock are sedimentary, metamorphic, and
igneous and the differences among them have to do with how they are formed.
Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other
fragments of material. Together, all these particles are called sediment. Gradually, the
sediment accumulates in layers and over a long period of time hardens into rock.
Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You
can often see sand, pebbles, or stones in the rock, and it is usually the only type that
contains fossils.
Examples of this rock type include conglomerate and limestone.
Metamorphic
Metamorphic rocks are formed under the surface of the earth from the metamorphosis
(change) that occurs due to intense heat and pressure (squeezing). The rocks that
result from these processes often have ribbonlike layers and may have shiny crystals,
formed by minerals growing slowly over time, on their surface.
Examples of this rock type include gneiss and marble.
Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and
hardens. Sometimes the magma cools inside the earth, and other times it erupts onto
the surface from volcanoes (in this case, it is called lava). When lava cools very quickly,
no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are
trapped in the rock during the cooling process, leaving tiny holes and spaces in the
rock.
Examples of this rock type include basalt and obsidian.
Here's a chart of some of the key characteristics that can help
you identify the rocks within these three main classes.
Crystals
Fossils
Small, flat
surfaces that
are shiny or
sparkly, like
tiny mirrors.
Imprints of
leaves, shells,
insects, or
other items in
the rock.
Gas
bubbles
Glassy
surface
Ribbonlike
layers
Sand or
pebbles
"Holes," like
Swiss cheese,
in the rock.
A shiny and
smooth
surface, like
colored glass.
Straight or
wavy stripes
of different
colors in the
rock.
Individual
stones,
pebbles, or
sand grains
visible in the
rock.